This invention relates to the economical removal of particles from combustion gas streams such as those of power plants, and, more particularly, to an approach for mutually optimizing the performance of the gas conditioning system and the electrostatic precipitator.
Conventional (non-nuclear) power plants that burn oil or coal produce unburned particulate matter that is entrained in the combustion gas stream. The particulate matter would, if permitted to flow up the exhaust stack and into the environment, deposit around and downwind of the plant in an unsightly, environmentally unacceptable manner. It is therefore standard practice to remove a large portion of the particulate matter from the combustion gas before the gas is exhausted, through the use of filters and/or electrostatic precipitators. The present invention relates to the use of electrostatic precipitators to remove the particulate matter.
The electrostatic precipitator applies an electrostatic charge to the particles in the gas stream. The combustion gas bearing the charged particles passes between oppositely charged electrode plates, causing the particles to be attracted to one of the electrodes by an electrostatic force. The particles adhere to the collecting electrode plates, and the mass of particles is periodically removed from the plates.
Under some circumstances, the electrical resistivity of the particles may be excessively high, so that the electrical resistivity of the particle mass adhering to the collecting electrode plates is also excessively high. The particle mass produces a high series electrical resistance that reduces the precipitation current that flows between the oppositely charged electrodes, in the manner of an insulator layer, thereby reducing the efficiency of the particle collection. A corona discharge in the collected layer of particulate matter often develops, giving the phenomenon its name of "back corona".
A number of different techniques have been developed to improve the efficiency of electrostatic precipitators. In one, conditioning agents are added to the combustion gas stream to modify and reduce the resistive character of the particles. In another, the electrostatic precipitator is placed on the hot side of the system combustion gas heat exchanger. At this temperature, the resistivity of the particulate is sufficiently low that it can be processed properly. In yet another approach, various types of special electrostatic precipitators have been devised.
One promising approach to improved efficiency of the electrostatic precipitator is to vary the duty cycle of the voltage applied to the precipitating elements of the electrostatic precipitator. Since the development of the corona effect is related to the capacitance of the particle mass on the collecting electrode, there is a time delay that is on the order of 0.1 to 2 seconds required to develop the adverse effects. It is known that the back corona effect may be reduced or avoided by energizing (applying a voltage between) the collection electrodes for a short period of time, and then deenergizing the electrodes before the back corona effect can develop. The electrodes are then reenergized and the process repeats. Experimental results have shown that both the collection efficiency of the particulate matter and also the power efficiency of the electrostatic precipitator can be improved by the use of such an intermittent voltage approach.
However, the success of the intermittent energization technique in achieving improved plant performance varies with the nature of the fuel being burned to form the combustion gas stream. There is a need for an approach to improving the operation of combustion gas cleanup systems, making the intermittent energization technique more broadly applicable, and achieving more nearly optimal system performance. The present invention fulfills this need, and further provides related advantages.